For the past several years, extensive research has been devoted to the development of metal-ceramic composites, such as aluminum reinforced with carbon, boron, silicon carbide, silica, or alumina fibers, whiskers, or particles. Metal-ceramic composites with good high temperature yield strengths and creep resistance have been fabricated by the dispersion of very fine (less than 0.1 micron) oxide or carbide particles throughout the metal or alloy matrix. However, this metal ceramic composite technology has not heretofore been extended to include intermetallic matrices. Prior art techniques for the production of metal-ceramic composites may be broadly categorized as powder metallurgical approaches, molten metal techniques, and internal oxidation processes.
The powder metallurgical type production of such dispersion-strengthened composites would ideally be accomplished by mechanically mixing metal powders of approximately 5 micron diameter or less with the oxide or carbide powder (preferably 0.01 micron to 0.1 micron). High speed blending techniques or conventional procedures such as ball milling may be used to mix the powder. Standard powder metallurgy techniques are then employed to form the final composite. Conventionally, however, the ceramic component is large, i.e., greater than 1 micron, due to a lack of availability, and high cost, of very small particle size materials since their production is energy intensive, time consuming, and costly in capital equipment. Furthermore, production of very small particles inevitably leads to contamination of the particles with oxides, nitrides, and materials from various sources such as the attritor (e.g., iron). The presence of these contaminants inhibits particulate-to-metal bonding which in turn compromises the mechanical properties of the resultant composites. Further, in many cases where the particulate materials are available in the desired size, they are extremely hazardous due to their pyrophoric nature.
Alternatively, it is known that proprietary processes exist for the direct addition of appropriately coated ceramics to molten metals. Further, molten metal infiltration of a continuous ceramic skeleton has been used to produce composites. In most cases, elaborate particle coating techniques have been developed to protect the ceramic particles from the molten metal during admixture or molten metal infiltration, and to improve bonding between the metal and ceramic. Techniques such as these have resulted in the formation of silicon carbide-aluminum composites, frequently referred to as SiC/Al, or SiC aluminum. This approach is only suitable for large particulate ceramics (e.g., greater than 1 micron) and whiskers, because of the high pressures involved for infiltration. The ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the intersticies. Such a technique is illustrated in U.S. Pat. No. 4,444,603, of Yamatsuta et al, issued Apr. 24, 1984. Because of the necessity for coating techniques and molten metal handling equipment capable of generating extremely high pressures, molten metal infiltration has not been a practical process for making metal-ceramic composites.
The presence of oxygen in ball-milled powders used in prior art powder metallurgy techniques, or in molten metal infiltration, can result in oxide formation at the interface between the ceramic and the metal. The presence of such oxides will inhibit interfacial binding between the ceramic phase and the matrix, thus adversely effecting ductility of the composite. Such weakened interfacial contact can also result in reduced strength, loss of elongation, and facilitated crack propagation. In addition, the matrix may be adversely effected, as in the case of titanium which is embrittled by interstitial oxygen.
Because of the above-noted difficulties with conventional processes, the preparation of metal-ceramic composites with submicron ceramic dispersoids for commercial applications has been extremely expensive.
Internal oxidation of a metal containing a more reactive component has also been used to produce dispersion strengthened metals, such as internally oxidized aluminum in copper. For example, when a copper alloy containing about 3 percent aluminum is placed in an oxidizing atmosphere, oxygen may diffuse through the copper matrix to react with the aluminum, precipitating alumina. This technique, although limited to relatively few systems since the two metals utilized must have a wide difference in chemical reactivity, has offered a feasible method for dispersion hardening. However, the highest possible level of dispersoids formed in the resultant dispersion strengthened metal is generally insufficient to impart significant changes in properties such as modulus, hardness, and the like. In addition, oxides are typically not wetted by the metal matrix, so that interfacial bonding is not optimum.
In recent years, numerous ceramics have been formed using a process referred to as self-propagating high-temperature synthesis (SHS), which involves an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders. Generally, the SHS process is ignited by electrical impulse, thermite, or spark. The SHS process involves mixirg and compacting powders of the constituent elements, and igniting the green compact with a suitable heat source. On ignition, sufficient heat is released to support a self-sustaining reaction, which permits the use of sudden, low power initiation of high temperatures, rather than bulk heating over long times at lower temperatures. Exemplary of these techniques are the patents of Merzhanov et al. In U.S. Pat. No. 3,726,643, there is taught a method for producing high-melting refractory inorganic compound by mixing at least one metal selected from groups IV, V, and VI of the Periodic System with a non-metal such as carbon, boron, silicon, sulfur, or liquid nitrogen, and locally heating the surface of the mixture to produce a local temperature adequate to initiate a combustion process. In U.S. Pat. No. 4,161,512, a process is taught for preparing titanium carbide by localized ignition of a mixture consisting of 80-88 percent titanium and 20-12 percent carbon, resulting in an exothermic reaction of the mixture under conditions of layer-by-layer combustion. These references deal with the preparation of ceramic materials, in the absence of a second non-reactive metallic phase.
U.S. Pat. No. 4,431,448 teaches preparation of a 35rd alloy by intermixing powders of titanium, boron, carbon, and a Group I-B binder metal, such as copper or silver, compression of the mixture, local ignition thereof to initiate the exothermic reaction of titanium with boron and carbon, and propagation of the reaction, resulting in an alloy comprising titanium diboride, titanium carbide, and the binder metal. This reference, however, is limited to the use of Group I-B metals such as copper and silver, as binders. As is set forth in the patent, products made by this method have low density, requiring subsequent compression and compaction.
Another class of materials which has seen considerable interest and development is intermetallic materials, especially intermetallics of aluminum such as the aluminides of titanium, zirconium, iron, cobalt, and nickel.
The need for the advanced properties obtainable with intermetallic materials is typified by their potential application to structures capable of withstanding high temperatures, such as turbine engines. In designing and operating turbine engines today and for the foreseeable future, there are two primary problem which demand solutions from the field of materials science. The first of these is the need to operate certain portions of the engine at higher gas and metal temperatures to improve operating efficiency and save fuel. The second problem is the need for lighter materials to decrease engine weight and engine operating stresses due to heavy rotating components, and to increase the operating life of disks, shafts, and bearing support structures. These latter structures require materials which are less dense than the nickel base superalloys they are intended to replace, but which possess roughly the same mechanical properties and oxidation resistance as those materialsin current usage.
The intermetallics are typically highly ordered compounds, in the sense that they possess regularly repeating (e.g., A B A B A B) atom sequencing. Intermetallic compounds are particularly suited to these needs because of two properties which derive from the fact that they possess ordered structures. Modulus retention at elevated temperature in these materials is particularly high because of strong A-B bonding. In addition, a number of high temperature properties which depend on diffusive mechanisms, such as creep, are improved because of the generally high activation energy required for self-diffusion in ordered alloys.
The formation of long range order in alloy systems also frequently produces a significant positive effect on mechanical properties, including elastic constants, strength, strain-hardening rates, and resistance to cyclic creep deformation. Finally, in the case of aluminides, the resistance to surface oxidation is particularly good because these materials contain a large reservoir of aluminum that is preferentially oxidized.
However, during metallurgical processing, one problem encountered is that these materials tend to form coarse grains, which degrade certain mechanical properties, the most important of which is ductility. Also, in many intermetallics the strong A-B bonding results in low temperature brittleness, although the exact mechanism of the ductile-brittle transition seems to be different for the different intermetallic compounds. It is thus necessary to address the problem of minimal low temperature ductility without destroying the inherent high temperature strength and stiffness. In the prior art it has generally been considered that these latter high temperature properties may only be retained by preserving the ordered structure, hence sacrificing low temperature ductility.
Since the early 1970's, the pace of work on ordered alloys and intermetallic compounds has slackened, as a result of lack of progress in improving either ductility or creep resistance of these otherwise very intriguing alloys.
Interest in utilizing ordered alloys for structural applications was reawakened in this country when researchers discovered that ductility and strength improvements could be achieved in TiAl and Ti.sub.3 Al based alloys using a combination of powder metallurgy and alloying techniques. Later work on the titanium aluminides utilized ingot metallurgy. The development of rapid solidification methods led to renewed interest in the iron and nickel aluminides. The replacement of cobalt in Co.sub.3 V by nickel, and then iron, led to a series of face-centered cubic Ll.sub.2 -type superlattices with greater ductility at ambient temperatures. Also, it has been reported in Japan that polycrystalline Ni.sub.3 Al can be made more ductile by adding small quantities of boron. Later, this work was confirmed and the critical composition range over which boron was beneficial was identified. (See U.S. Pat. No. 4,478,791 of Huang et al, assigned to General Electric.) These discoveries, together with the national search for replacements for strategic metals, such as cobalt and chromium, and the need to develop energy-efficient systems, have in the past few years or two stimulated much additional work; largely in the area of improving low temperature ductility and increasing high temperature strength.
Despite these efforts, little progress has been made in developing practical intermetallic compositions that have sufficiently improved low temperature ductility while maintaining high temperature strength.